1. Technical Field
The present application claims the priority of Japanese Patent Application No. 2009-140846 filed in Japan on Jun. 12, 2009, the contents of which are hereby incorporated by reference.
The present invention relates to an ink composition for an organic EL thin film (a solution containing an organic EL material), a method for forming an organic EL thin film, an organic EL thin film, and an organic EL element. The present invention relates to, for example, a solution containing an organic EL material, for use in the formation of an organic thin film constituting an organic EL element by a coating method.
2. Background Art
Organic electroluminescence (EL) elements refer to elements using the principle that holes and electrons injected respectively from a positive electrode and a negative electrode by the application of an electric field are recombined to cause an organic material to produce luminescence. Since C. W. Tang, et al., at Eastman Kodak Company reported a low-voltage driving organic EL element of a laminate-type element, research has been carried out actively on organic EL elements including an organic material as a constituent material.
Tang, et al. have proposed a stacked structure using tris(8-quinolinol)aluminum for a light-emitting layer and a triphenyldiamine derivative for a hole transporting layer. The advantages of the stacked structure include the ability to increase the injection efficiency of holes into the light-emitting layer, the ability to increase the generation efficiency of excitons generated by recombination through the blocking of electrons injected into a negative electrode, and the ability to trap excitons generated in the light-emitting layer. Two-layer type structures of a hole transporting (injecting) layer and an electron transporting light-emitting layer, three-layer type structures of a hole transporting (injecting) layer, a light-emitting layer, and an electron transporting (injecting) layer, or the like are well-known as the element structure of an organic EL element as in this example. In order to increase the recombination efficiency of injected holes and electrons, various devises have been made for the element structure and formation method in the case of these elements of stacked structure type.
While high-molecular materials and low-molecular materials are known as organic EL materials, the development of low-molecular organic EL materials has been advanced in view of simplicity in synthesis and of possible high-purity purification through sublimation purification. Furthermore, among the low-molecular organic EL materials, excellent organic EL materials in terms of efficiency, lifetime, and color purity have been reported, and put into practical use.
For the deposition of the low-molecular organic EL material into a thin film, a vacuum deposition method is adopted, so that the low-molecular organic EL material is sublimated with favorable thermal stability and deposited onto a substrate to achieve a high-performance organic EL element. However, the vacuum deposition method has the problem of requirements for high-vacuum equipment and a complicated production process, thus, deposition through a wet method such as spin coating or ink jet has been desired instead of the vacuum deposition method.
On the other hand, organic electroluminescence elements using, as a luminescent material, a polymer such as poly(p-phenylenevinylene) (PPV) or poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) have been published (for example, as described in Nature, 347, p. 539, (1990) (Non-Patent Document 1) and Appl. Phys. Lett. 58, p. 1982, (1991) (Non-Patent Document 2)).
Thus, simple element deposition techniques with wet deposition (coating deposition) have been developed actively with the use of high-molecular luminescent materials. The solution containing an organic EL material for the wet deposition needs to dissolve the desired organic EL material and to have a desired viscosity. For example, in the case of deposition through a spin coating or ink-jet method, the viscosity of the solution needs to be several cps or more. As techniques for modifying the viscosity, techniques for ink compositions with an alcohol solvent added are disclosed (for example, as described in Japanese Patent Laid-Open Publication No. 2008-124157 (Patent Document 1) and Japanese Patent Laid-open Publication No. 2007-527624 (Patent Document 2)).